Heat-expandable microspheres, which comprise a shell of a thermoplastic resin and a blowing agent encapsulated therein, are generally called heat-expandable microcapsules. The thermoplastic resin usually includes vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic acid ester copolymers. The blowing agent mainly employed includes hydrocarbons, such as isobutane and isopentane. (Refer to Patent Reference 1).
An example of such heat-expandable microcapsules already is disclosed as heat-expandable microspheres comprising a shell of a copolymer produced by polymerizing (I) a nitrile monomer and (II) a carboxyl-group-containing monomer. (Refer to Patent Reference 2). Although the heat-expandable microcapsules have good heat resistance, a higher grade of heat resistance is required to meet the recent demand for better microcapsule properties.
A variant of similar heat-expandable microcapsules comprising a shell of a copolymer produced by polymerizing (I) acrylonitrile, (II) a carboxyl-group-containing monomer and (III) a monomer containing a group reactive to the carboxyl group have been disclosed. (Refer to Patent Reference 3.) The heat-expandable microcapsules have good heat resistance. On the other hand, the carboxyl group and the group reactive to the carboxyl group, which are uniformly distributed in the copolymer, may excessively promote crosslinking when the microcapsules are heated and expanded. The hollow particulates produced by heating and expanding the microcapsules have good solvent resistance owing to the promoted cross-linking in the heating and expanding, while the solvent resistance is required to be improved to higher grade.
Patent Reference 4 describes a variant of heat-expandable microcapsules which comprise a shell of a copolymer having a polymethacrylimide structure produced from methacrylonitrile and methacrylic acid and a blowing agent being encapsulated therein. The heat-expandable microcapsules also have good heat resistance. However, the shell has poor gas-barrier performance and solvent resistance, because the high ratio of methacrylonitrile in the copolymer resin constituting the shell decreases the crystallinity of the copolymer resin. The poor gas-barrier performance remarkably deteriorates the heat resistance and expansion performance of the microcapsules employed in resin molding where the microcapsules are held in a high-temperature environment for a long time.